Network Slice Admission Control Function

Description

The Network Slice Admission Control Function (NSACF) is a critical control plane network function introduced in 3GPP Release 17 as the central entity responsible for executing Network Slice Admission Control (NSAC) policies. It is a logical function that can be deployed as a standalone network element or collocated with other control plane functions. The primary role of the NSACF is to manage and enforce the maximum allowed number of User Equipments (UEs) registered to each network slice instance (NSI). It does this by maintaining accurate, real-time counters for UE registrations and deregistrations on a per-slice basis. The AMF, which handles UE registration requests, interacts with the NSACF to determine if a UE can be admitted to a requested slice.

The NSACF operates through a set of service-based interfaces, primarily exposing services to the AMF. When a UE initiates a registration procedure including a Network Slice Selection Assistance Information (NSSAI), the AMF invokes a service operation on the NSACF (e.g., Nnsacf_NSAC_Control service) to request an admission check. The NSACF receives this request, which includes the slice identifier (S-NSSAI), and checks its internal database or policy repository for the configured maximum UE limit and the current registration count for that slice. Based on this check, it returns an admission control decision (allow or reject) to the AMF. If allowed, the NSACF increments its internal counter. Conversely, when a UE deregisters or its registration context is released, the AMF notifies the NSACF, which then decrements the corresponding counter.

Architecturally, the NSACF is a stateful function that must ensure data consistency and reliability. It may interact with a Unified Data Repository (UDR) to persistently store slice admission control policy data and potentially the registration counts for recovery purposes. The NSACF's design also considers scalability and high availability, as it becomes a central point for slice admission control across the entire network. Its functionality is tightly integrated with the overall network slicing management system, including the Network Slice Selection Function (NSSF) and the Policy Control Function (PCF), to ensure a cohesive slice management and policy enforcement framework.

Purpose & Motivation

The NSACF was created to provide a dedicated, scalable, and centralized point for enforcing Network Slice Admission Control policies. Prior to its introduction, there was no standardized function to manage the scale of network slices in terms of connected devices. Relying on individual network functions like the AMF to locally enforce such limits would be inefficient, inconsistent, and difficult to manage network-wide. The NSACF solves this by centralizing the logic and state for slice admission, ensuring a single source of truth for slice registration counts across all AMFs in the network.

This centralization is crucial for several reasons. First, it guarantees consistent policy enforcement regardless of which AMF a UE attaches to, which is essential in a cloud-native, distributed core network. Second, it simplifies operations and policy management for network operators, as they can configure and update slice capacity limits in one logical function. Third, it enables advanced features like dynamic capacity adjustments and integration with network analytics. The NSACF's creation was a direct response to operator requirements for robust commercial slicing, where guaranteeing a slice is not oversubscribed is as important as guaranteeing its performance, making it a cornerstone for slice-as-a-service business models.